Pare and cook potatoes in boiling salted water until tender. Drain and press through ricer. Add butter or margarine, milk, salt, pepper, onion and egg yolks. Blend thoroughly and fold in stiffly beaten egg whites. Pile in greased 1 1/2 quart casserole and bake in hot oven (450 F.) about 15 minutes until lightly browned. Garnish with strips of pimiento. Yield: 8-10 servings.